Project 6 will use the acoustic startle reflex to assess the behavioral consequences of early life stressors at various stages of development. The acoustic startle reflex has a number of advantages that makes it a useful measure to assess the long-term behavioral consequences of early maternal separation. Its non-zero baseline allows both excitatory and inhibitory effects to be measured. The startle reflex can be elicited by different types of sensory stimuli and shows several forms of plasticity, such as habituation, sensitization, prepulse inhibition, and modulation by fear or anxiety. In addition, acoustic startle has a well-defined neural pathway that allows one to determine where these various forms of plasticity alter neural transmission so as to affect the behavior. Because of this, it should be possible to more fully determine the neural mechanisms that allow early life stressors to produce permanent changes in behavior. The neural pathway that mediates the acoustic startle reflex receives a direct input from the amygdala and the bed nucleus of the stria terminalis (BNST). Certain phenomena, such as the elevation of startle in the presence of a conditioned fear stimulus (the fear-potentiated startle effect), depend on projections from the central nucleus of the amygdala to the startle pathway. Other phenomena, such as the elevation of startle produced by unconditioned anxiogenic events such as exposure to a very bright light or intraventricular infusion of corticotropin releasing factor (CRF), depend on projections from the BNST to the startle pathway. Because repeated maternal separation leads to increased expression of corticotropin releasing hormone messenger RNA (CRF mRNA) in the central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST), brain structures critically involved in fear, anxiety, and regulation of the HPA axis, Project 6 will evaluate whether postnatal environmental events such as handling and maternal separation will alter either the amplitude or plasticity of the startle reflex, and, if so, how these changes might relate to changes in the amygdala or BNST. In Aims 1-3 baseline startle amplitude, startle habituation, prepulse inhibition, fear- potentiated startle, and light-enhanced startle will be measured at various times in development and in adulthood in HMS 180, HMS 15 and AFR rats. These studies will assess whether early life stressors produce permanent changes in attention, anxiety or fear. In Aims 4 and 5 we will measure how CRF given intraventricularly or yohimbine given systemically, or directly into the bed nucleus of the stria terminalis will affect startle amplitude at various points in development and in adulthood in HMS180, HMS15 and AFR rats. These studies should provide a direct test of the behavioral consequences of a change in regulation of CRF or catecholamines flowing early life stressors.